Conventional methods for controlling a switching regulator have employed voltage control in which only an output voltage is fed back.
In order to improve frequency characteristics of switching regulators using such a voltage control method, current control has been used (refer to Patent Document 1, for example), in which the output voltage and an output current are fed back. Further, as a similar method, PID control has been used, in which stability is improved using an approximate differentiator from the output voltage feedback.
The switching regulator using the voltage control method controls an inductor current and generates a certain output voltage at a predetermined constant voltage by charging an output smoothing capacitor with the inductor current. In such a switching regulator, the output voltage is fed back and the output current is generated in order to control the output voltage, so that the switching regulator has characteristics of a secondary resonance frequency as inherent characteristics of the switching regulator. The resonance frequency characteristics reduce stability of a control loop and complicate a structure of the switching regulator. Accordingly, it is necessary to reduce a gain of the control loop in order to maintain the stability and this poses a problem in that transient response characteristics are reduced.
Further, the current control method in which the output voltage and the output current are fed back has been used in order to improve the frequency characteristics of the switching regulator of an output voltage feedback type using the voltage control method. The output current is fed back to control the output current, so that the switching regulator has characteristics of a primary frequency and control is readily performed. Accordingly, it is possible to increase the gain of the control loop, so that it is possible to improve the transient response characteristics of the switching regulator.    Patent Document 1: Japanese Laid-Open Patent Application No. 2006-33958
However, in the current control method, the current is converted to the voltage and fed back, so that a current sensing resistor is necessary. When a resistance value of the current sensing resistor is large, efficiency of the switching regulator is reduced, so that it is necessary to use a resistor with several dozens of mΩ for the current sensing resistor. However, such a resistor has been expensive. Further, since a sense voltage by the current sensing resistor is a minute voltage, there has been a problem in that the minute voltage is likely to be affected by a noise. Further, as a method without the use of such a current sensing resistor, a method using on-resistance of a driver transistor has been employed (a drain voltage of the driver transistor is used). This method is more advantageous than the current sensing resistor in terms of cost and efficiency because no resistor is used.
However, due to switching of the driver transistor, it is difficult to adjust timing for detecting the drain voltage when the driver transistor is switched on. Further, because of generation of a switching surge of the driver transistor, it is impossible to detect a voltage immediately after the driver transistor is switched on and this has been problematic in that a delay of the detection timing is generated. Moreover, since a minute voltage is used, there has been a problem in that the minute voltage is very likely to be affected by a noise in the same manner as in the method using the current sensing resistor. In addition, in the switching regulator using the current control method, when an on-duty cycle of PWM control exceeds 50%, a subharmonic resonance is generated. In order to remove the subharmonic resonance, a slope compensation circuit is required. Such a slope compensation circuit has a complicated structure and adjustment thereof has been very difficult.
Moreover, in the PID control method in which the stability is improved using the approximate differentiator from the output voltage feedback, theoretically, a differential value of the output voltage is fed back. In other words, a value close to a difference between the output current and the inductor current is fed back. Accordingly, it is possible to have frequency characteristics similar to those in the current control method. However, in the switching regulator of the PID control method, the output voltage is superposed by a large high-frequency noise due to ESR from smoothing capability or a surge voltage from an output node of the switching regulator. From the structure where the differentiator is used, it is apparent that such a high-frequency noise may become a factor in malfunction.
In this manner, although the current control method has a merit of good response characteristics, it has many demerits for the voltage control method. The PID control method uses the differentiator and thus is subject to noise. On the basis of these facts, currently, many switching regulators use the voltage control method.